Radiopharmaceuticals are drugs containing a radionuclide. Radiopharmaceuticals are used routinely in nuclear medicine for the diagnosis or therapy of various diseases. They are typically small organic or inorganic compounds with a definite composition. They can also be macromolecules, such as antibodies or antibody fragments that are not stoichiometrically labeled with a radionuclide. Radiopharmaceuticals form the chemical basis for the diagnosis and therapy of various diseases. The in vivo diagnostic information can be obtained by intravenous injection of the radiopharmaceutical and determination of its biodistribution using a gamma camera or a PET camera. The biodistribution of the radiopharmaceutical typically depends on the physical and chemical properties of the radiolabeled compound and can be used to obtain information about the presence, progression, and state of disease.
Radiopharmaceuticals can generally be divided into two primary classes: those whose biodistribution is determined exclusively by their chemical and physical properties, and those whose ultimate distribution is determined by their receptor binding or other biological interactions. The latter class is often described as being target-specific.
Recently, much effort has been expended on the discovery and development of radiopharmaceuticals for diagnostic imaging which contain positron emitting isotopes. Positron emitting isotopes include 82Rb, 124I, 11C, 13N, and 18F, among others. These isotopes decay by the emission of a positron from the nucleus. A positron is a particle that has an equivalent mass of an electron, but a corresponding positive charge. The positron, after ejection from the nucleus, travels until it encounters an electron, and the reaction of the two results in a physical annihilation of the masses. Energy is released in opposing directions at a value of 511 kEv, and because the annihilation has no angular momentum, the photons are projected from the point of annihilation approximately 180 degrees apart, allowing for precise determination of a line along which the said decomposition occurred. This property results in exquisite sensitivity and resolution, and allows for superb image reconstruction and quality.
An advantage of the carbon, nitrogen and fluorine isotopes is that they may be incorporated into small organic molecules, such as known or investigational pharmaceuticals that could be used to determine biodistribution of the agent, as well as diagnose the presence, absence or extent of disease. They may conveniently be inserted into these molecules by a variety of methods known to organic chemists and radiochemists ordinarily skilled in the art. Widespread use in investigational research has been made of 11C-methyl iodide (11CH3I), methylating an alcohol or an amine to produce the corresponding ether or alkyl amine. These compounds are then appropriately sterilized, formulated and injected into a subject.
The primary drawback to the widespread use of many PET radiopharmaceuticals is the relatively short half lives associated with many of the isotopes. Rubidium-82, carbon-11, and nitrogen-13 have half-lives of 1.27, 20.3, and 9.97 minutes, respectively. Rubidium is administered as the chloride salt from a 82Sr—82Rb generator, and is not synthetically modified or manipulated. Nitrogen-13 is typically administered as ammonia (13NH3) produced in a cyclotron adjacent to an imaging center with appropriate proximity to a camera. Both 11C- and 13N-based reagents have been used in the radiolabeling of imaging agents. Significant engineering and logistical challenges need to be met to allow for the use of the compounds as radiopharmaceuticals given the short half life and the necessary time to accomplish the required reactions and purification prior to formulation and administration of the drug.
Correspondingly longer-lived positron emitting isotopes may be incorporated into new radiotracers for imaging. These include the aforementioned 131I and 18F, with half-lives of 4.2 days and 107.9 minutes, respectively. The most prevalent use of late has been 18F, as the decay is entirely through the emission of positrons and has a favorable half life. The approximate two hours allows for synthetic incorporation into a molecule, purification and subsequent distribution from a centrally located radiopharmacy, obviates the requirement/investment in either an on-site cyclotron or the monthly purchase of a 82Sr—82Rb generator.
During the course of manufacture, formulation, release, and delivery of doses, the isotope typically decays at a zero-order rate dictated by the physics of each particular isotope. However, this decay can also trigger chemical decay of the dose, by radiolysis. This can propagate via radical reaction and seriously diminish the quality of the composition.
Decomposition of the radiopharmaceutical composition prior to or during administration can dramatically decrease the targeting potential and increase the toxicity of the therapeutic radiopharmaceutical composition. Thus, in some cases, it is important to ensure that the radionuclide is linked to the targeting moiety, and to further ensure that specificity of the targeting agent is preserved.
Radiolysis is caused by the formation of free radicals such as hydroxyl and superoxide radicals (Garrison, W. M. Chem. Rev. 1987, 87, 381-398). Free radicals are very reactive towards organic molecules. The reactivity of these free radical towards organic molecules can affect the solution stability of a radiopharmaceutical composition. Stabilization of the radiopharmaceutical composition is a recurrent challenge in the development of target-specific radiopharmaceuticals, and radical scavengers are often employed as a stabilizer to minimize radiolysis of the radiolabeled molecules. Some stabilizers are “radical scavenging antioxidants” that readily react with hydroxyl and superoxide radicals. The stabilizing agent for radiopharmaceutical compositions may advantageously possess the following characteristics: low or essentially no toxicity when it is used for human administration, low or essentially no interference with the delivery or receptor binding of the radiolabeled compound to the target cells or tissue(s), and/or the ability to stabilize the radiopharmaceutical for a reasonable period of time (e.g., during the preparation, release, storage and transportation of the radiopharmaceutical).
Radical scavengers such as ascorbic acid have been used to stabilize 99mTc (DeRosch, et al, WO95/33757) and 186/188Re (Anticancer Res. 1997, 17, 1783-1796) radiopharmaceuticals. U.S. Pat. No. 5,393,512 discloses the use of ascorbic acid as a stabilizing agent for 186Re and 131I-labeled antibodies or antibody fragments. U.S. Pat. Nos. 5,093,105 and 5,306,482 disclose the use of ascorbic acid as an antioxidant for 99mTc radiopharmaceuticals.
Several strategies have been developed for the use of antioxidants such as ascorbic acid to terminate decay pathways prior to significant damage occurring. Ascorbic acid has been used in various pharmaceutical and radiopharmaceutical compositions. Unlike other buffering agents such as succinic acid and aminocarboxylates, ascorbic acid contains no amino or carboxylic groups. PCT/US94/06276 discloses stabilizing agents such as ascorbic acid and water soluble salts and esters of ascorbic acid.
U.S. Pat. No. 6,066,309 discloses the use of ascorbic acid and derivatives thereof in stabilizing radiolabeled proteins and peptides against oxidative loss of radiolabels and autoradiolysis. In some cases, ascorbic acid is added after radiolabeling, including any required incubation period, but prior to patient administration. In addition, derivatives of ascorbic acid are defined as salts of ascorbic acid, esters of ascorbic acid, or mixtures thereof.
Although the use of ascorbic acid/ascorbate as a stabilizer has been disclosed for a variety of diagnostic and therapeutic radiopharmaceutical compositions (see, e.g., Deausch, E. A. et al./U.S. Pat. No. 5,384,113/1995; Vanderheyden, J.-L., et al./U.S. Pat. No. 5,393,512/1995; Flanagan, R. J. and Tartaglia, D./U.S. Pat. No. 5,093,105/1992; Tartaglia, D. and Flanagan, R. J./U.S. Pat. No. 5,306,482/1994; Shochat, D. et al./U.S. Pat. No. 5,961,955/1999; and Zamora, P. O. and Merek, M. J./U.S. Pat. No. 6,066,309/2000), there has been little or no disclosure regarding the use of ascorbate within a specified range of pH to enhance the antioxidant action of the compound for clinical applications.
While significant use of antioxidants such as ascorbic acid have been exemplified in the literature, little attention has been paid to the state of the antioxidant, e.g., as when adding it into a buffered solution for stability studies at low pH or at higher pH for material suitable for injection.
Material suitable for injection in humans may be selected to have a pH higher than 4.0 to reduce the risk of localized irritation and pain associated with a strongly acidic solution at an injection site. Typically, solutions for injection have been buffered by phosphate (phosphate buffered saline (PBS)) in the pH range of 6-8. However, the employment of ascorbic acid/ascorbate in buffered solutions at typical biological pH ranges (6-8) often exhibits a lower ability to stabilize radiopharmaceutical solutions. Conversely, while previous work may demonstrate stability of radiopharmaceutical preparations using ascorbic acid at low pH values (2-3), such formulations are generally not suitable for use in animal models or humans due to localized reactions, as noted above. In addition, previous work may set forth a broad acidic pH range for the ascorbic acid than is useful, or specify no particular range at all. To date, it is believed that there has been little guidance for the skilled artisan in selecting pH when using ascorbic acid for clinical applications of radiopharmaceuticals.
Accordingly, improved compositions and methods are needed.